The present invention relates to a reflection type liquid crystal display device, and to a method for driving a reflection type color liquid crystal device.
A display equipped in a portable information terminal should be of the low power consumption type. Thus, a reflection type liquid crystal device, which does not need back light, is most suited for such an application. However, the dominating conventional reflection type liquid crystal device is of the monochromatic display type. Therefore, good reflection type color display has not been obtained yet.
Full-scale development of the reflection type color liquid crystal device was started about the middle of 1980""s. There had been only a superficial understanding before then. For example, the reflection type color display could be obtained by replacing the backlight unit of the transmission type color liquid crystal device.
However, when the liquid crystal device is actually made of such a configuration, it has been found that only very dark display could be achieved and that the device of such a configuration was not practical. There have been three causes. First, the polarizing plate has discarded more than half of the quantity of light. Second, the color filter has discarded more than ⅔ of the quantity of light. Finally, there has been the problem of a parallax. Parallax has been the inevitable problem in the case of TN (twisted nematic) mode and STN (supertwisted nematic) mode. This is because of the fact that two polarizing plates are necessarily used in these modes and thus there is a distance created between the reflector plate and the liquid crystal layer, which cannot be disregarded, unless the polarizing plates are not built in a cell. Incidentally, the problem of the parallax referred to herein is not the problem of a double-image in the display which has occurred even in the conventional reflection type monochromatic liquid crystal device. Namely, the parallax is a problem that occurs in and is peculiar to the reflection type color liquid crystal device.
Problems of parallax will be described hereunder by referring to the accompanying drawings. FIGS. 7(a) and 7(b) are sectional views of a reflection type color liquid crystal device utilizing TN mode or STN mode. This liquid crystal device consists of an upper polarizing plate 1. an upper glass substrate 2, a liquid crystal layer 3, a lower glass substrate 4, a lower polarizing plate 5, an optical reflector plate 6 and red-green-blue (RGB) three-color filter 7. Additionally, there are transparent electrodes, an orientation film and insulating film between the upper and lower glass substrates. These composing elements are, however, unnecessary for describing the problem of a parallax. Thus, the drawing thereof is omitted.
Meanwhile, there are two problems with the parallax. First, one of the two problems is color cancellation. As shown in FIG. 7(a), an observer 32 watches reflection light 31 having passed through a green filter. This reflection light has been mixed with incident light 30 which has been incident thereon through the red-green-blue color filter and has been diffused and reflected by the reflector plate. If the thickness of the lower glass substrate 4 is sufficiently large in comparison with the pitch of filter elements of the color filter 7, a light ray propagated through any color filter element is mixed with the reflection light 31 with same probability.
However, the light of any wavelength, which has passed through the paths xe2x80x9cred to greenxe2x80x9d and xe2x80x9cblue to greenxe2x80x9d, is inevitably absorbed by one of the filter elements. Thus, only the light having passed through the path xe2x80x9cgreen to greenxe2x80x9d remains. This is the same with reflection light rays having transmitted from the blue and red filter elements. Consequently, the brightness of displayed white is decreased to ⅓ that of the case in which there is no parallax.
A second problem is that the displayed colors become dark. FIG. 7(b) illustrates a green displaying state. Further, the cross hatched portion of the liquid crystal layer 3 indicates that such a portion is in a non-illuminated state (namely, a dark state). Incident light 30 passes through each of red, green and blue dots with the same probability. However, ⅔ of the quantity of the incident light 30 are absorbed by the red and blue dots which are in an off-state. Furthermore, after diffused by the reflector plate 6 and mixed with the reflection light, ⅔ of the quantity of such light are absorbed by the red and blue dots which are put into the off-state again. Then, the light reaches the observer 32. Therefore, the brightness of the display of green is obtained by subtracting the quantity of light, which is absorbed by the green filter element, from {fraction (1/9)} of the brightness of the displayed white (namely, {fraction (1/9)} of the brightness of the display of whitexe2x80x94the quantity absorbed by the green filter element), and thus becomes very dark.
As is understood from the foregoing description, it is very difficult to apply TN mode and STN mode, in each of which the problem of the parallax occurs, to the reflection type color liquid crystal device.
Thus, hitherto, attempts have been made to obtain bright reflection type color displays by changing the liquid crystal mode.
For example, according to the article by Mr. Tatsuo Uchida et al. (IEEE Transactions on Electron Devices, Vol ED-33, No. 8, pp. 1207-1211 (1986)), as illustrated in FIG. 2 therein, the comparisons of the brightness among various liquid crystal modes are made. As a consequence, PCGH (Phase-Change type Guest Host) mode, which does not need polarizing plates, is employed. Furthermore, in the case of the Japanese Unexamined Patent Publication No. 5-241143 Official Gazette, PDLC (Polymer-Dispersed type liquid crystal) mode, which does not require the polarizing plates, is employed so as to realize a reflection type color liquid crystal device.
In the case of using the liquid crystal mode which does not need polarizing plates, there are obtained the merits in that the absorption of light by the polarizing plate is eliminated and that the problem of the parallax can be settled completely by providing a reflector plate in such a manner so as to be adjacent to the liquid crystal layer.
However, on the other hand, in the case of the liquid crystal modes requiring no polarizing plates, the contrast is usually low. Further, especially, PCGH mode has encountered the problem in that halftone display cannot be performed owing to the presence of a hysteresis in the voltage-transmittance characteristic. Furthermore, these liquid crystal modes, by which foreign substance is added to the liquid crystal, have encountered the problem in degradation in the reliability.
Therefore, TN mode and STN mode, which have been widely used heretofore and achieved satisfactory results, are the best modes to be employed, if can be used even in the aforementioned conditions.
Additionally, hitherto, attempts have been made to obtain a bright reflection type color display by using the bright color filter. An example of this is disclosed in the Japanese Unexamined Patent Publication NO. 5-241143 Official Gazette. In this case, a reflection type color liquid crystal device is configured by using a color filter consisting of filter elements respectively corresponding to the subtractive primaries, namely, yellow, cyan and magenta.
This method has profound effects in obtaining bright displays but has faced the following problems.
Namely, ordinary color liquid crystal devices perform the additive mixing process by using a set of small color points, and thus use the color filter consisting of filter elements respectively corresponding to the additive primaries, namely, red, green and blue. However, according to the aforementioned Official Gazette, the additive mixing process is performed by using the color filter corresponding to the subtractive primaries. Thus, the degree of the color saturation of the displayed color is low, and the clear display cannot be achieved. Incidentally, according to the same Official Gazette, PDLC mode, which does not use polarizing plates, is employed. In addition, no parallax is caused as a result of providing the reflection plate at a place adjacent to the liquid crystal layer across the color filter.
Accordingly, an object of the present invention is to provide a reflection type liquid crystal device which can obtain a bright display by using a color filter consisting of filter elements respectively corresponding to Yellow, cyan and magenta, and can display bright colors in comparison with the conventional devices by making good use of parallax.
In accordance with the present invention, there is provided a preferable embodiment of a reflection type color liquid crystal device, which has a liquid crystal cell, in which a liquid crystal layer is held between a first substrate provided with a transparent electrode and a second substrate provided with a transparent electrode and a color filter, wherein the liquid crystal cell is placed between a pair of polarizing plates, wherein an optical reflector plate is formed outside one of the substrates, and wherein the aforementioned color filter comprises filter elements respectively corresponding to subtractive primaries that are yellow, cyan and magenta color elements, and wherein the lowest transmittance of the color filter, which correspond to each of the color elements, is not less than 10%.
Thus, the reflection type color liquid crystal device, which realizes a bright and clear display, can be obtained by setting the lowest transmittance of the color filter, which corresponds to each of the color elements, as being not less than 10%. Generally, the range of wavelengths from 400 to 770 nm is referred to as the visible region. Especially, the human visual sensitivity is high in the range of wavelengths from 450 to 660 nm. Therefore, a bright and clear display can be attained by setting the transmittance of the color filter correspondingly to each of all of the color elements as above described.
Further, regarding the color filter, the lowest transmittance thereof corresponding to each of the color elements in the visible region is set at a value within a range of 15% to 25%.
Further, a preferable embodiment of the reflection type color liquid crystal device of the present invention is adapted so that the spectrum representing a transmission characteristic of the aforesaid yellow filter element intersects with the spectrum representing a transmission characteristic of the aforesaid magenta filter element at a wavelength being close to 500 nm, wherein the spectrum representing a transmission characteristic of the aforesaid cyan filter element intersects with the spectrum representing a transmission characteristic of the aforesaid magenta filter element at a wavelength being close to 600 nm, and wherein these two points of intersection are present in a range where a transmittance is not less than 30%. More preferably, the aforesaid color filter is formed so that the two points of intersection are present in a range where the transmittance is 35% to 60%.
Moreover, a preferable embodiment of the reflection type color liquid crystal device of the present invention is adapted so that a distance between the optical reflector plate and the color filter is set as being larger than the pitches of dots formed by the aforesaid electrode. More preferably, the distance between the optical reflector plate and the color filter is set as being twice to three times each of the dot pitches. Incidentally, it is not desirable that the thickness of the glass substrate becomes larger than 0.7 mm. This is because the double image of the display is conspicuous. With such a configuration, the color saturation of the displayed colors is enhanced by the subtractive mixing which utilizes the parallax.
Furthermore, a preferable embodiment of the reflection type color liquid crystal device of the present invention is adapted so that a pixel electrode is placed on one of the aforesaid substrates in such a manner as to be formed like a matrix, and that a switching element is formed by being connected to the aforesaid pixel electrode. Thus, a high-precision display reflection type color liquid crystal device can be obtained.
Furthermore, a preferable embodiment of a reflection type color liquid crystal device, which has a liquid crystal cell, in which a liquid crystal layer is held between a first substrate provided with a reflector electrode and a second substrate provided with a transparent electrode and a color filter, wherein the liquid crystal cell is placed between a pair of polarizing plates, wherein an optical reflector plate is formed outside one of the substrates, and wherein the aforementioned color filter comprises filter elements respectively corresponding to subtractive primaries that are yellow, cyan and magenta color elements, and wherein the lowest transmittance of the color filter, which correspond to each of the color elements, is not more than 10% in a visible region.
Thus, the reflection layer (namely, the reflector electrode) is placed close to the liquid crystal layer.
Generally, the range of wavelengths from 400 to 770 nm is referred to as the visible region. Especially, the human visual sensitivity is high in the range of wavelengths from 450 to 660 nm. Therefore, a bright and clear display can be attained by setting the transmittance of the color filter correspondingly to each of all of the color elements in a visible range (400 to 770 nm) as above described.
Moreover, a reflection type color liquid crystal device can be obtained by placing an optical diffusion plate between the second substrate and the polarizing plate.
Furthermore, regarding the color filter, the lowest transmittance thereof corresponding to each of the color elements in the visible region is set at least 20% or more and, more preferably, 30% or more.
On the other hand, a preferable embodiment of the reflection type color liquid crystal device of the present invention is adapted so that the spectrum representing a transmission characteristic of the aforesaid yellow filter element intersects with the spectrum representing a transmission characteristic of the aforesaid magenta filter element at a wavelength being close to 500 nm, wherein the spectrum representing a transmission characteristic of the aforesaid cyan filter element intersects with the spectrum representing a transmission characteristic of the aforesaid magenta filter element at a wavelength being close to 600 nm, and wherein these two points of intersection are present in a range where a transmission is not less than 30%. More preferably, the aforesaid color filter is formed so that the two points of intersection are present in a range where the transmittance is 35% to 60%.
By using such a color filter, a bright display reflection type color liquid crystal device can be obtained.
Moreover, a preferable embodiment of the reflection type color liquid crystal device of the present invention is adapted so that a distance between the optical reflector plate and the color filter is set as being larger than the pitches of dots formed by the aforesaid electrode. More preferably, the distance between the optical reflector plate and the color filter is set as being twice to three times each of the dot pitches. Incidentally, it is not desirable that the thickness of the glass substrate becomes larger than 0.7 mm. This is because the double image of the display is conspicuous.
With such a configuration, the color saturation of the displayed colors is enhanced by the subtractive mixing which utilizes the parallax.
Furthermore, a preferable embodiment of the reflection type color liquid crystal device of the present invention is adapted so that a pixel electrode is placed on one of the aforesaid substrates in such a manner so as to be formed like a matrix, and that a switching element is formed by being connected to the aforesaid pixel electrode. Thus, a clearer display reflection type color liquid crystal device can be obtained.
Additionally, correspondingly to a preferable embodiment of the reflection type color liquid crystal device of the present invention, there is provided a method of driving the aforementioned reflection type color liquid crystal device, wherein, when displaying any color with the exception of black, a plurality of dots are turned on or partially turned on among 3 dots corresponding to each of the aforesaid yellow, cyan and magenta. In other words, only in the case of displaying black, 3 dots are non-illuminated. In the case of displaying 3 colors such as red, green and blue, 2 dots are turned on. In the case of other colors, all of 3 dots are turned on or partly illuminated. Incidentally, the turning-on is defined as an operation of changing the liquid crystal device into a bright state. Further, non-illuminating is to put the liquid crystal device into a dark state. Furthermore, an operation of partly turning on is to put the liquid crystal device into an intermediate state between the bright and dark states. With such a configuration, the reflection type color liquid crystal device can realize an extremely bright display. Further, the intermediate display can be achieved. Thus, the present invention has an advantage in that full color display can be attained.
Furthermore, by mounting the reflection type color liquid crystal device on the electronic devices, lower power consumption electronic device can be obtained.